The present invention relates to an electric power converter and particularly to the electric power converter which is suitably used as a DCxe2x80x94DC converter or switching regulator through switching of input voltage.
A regulator based on voltage drop was used to supply stable d.c. voltage to the load in an electronic apparatus or the like in the prior art. This regulator, however, features a poor power efficiency and produces much heat. To solve this problem, a switching regulator is often used instead of the regulator based on voltage drop.
The switching regulator causes the switch element to be switched at a high frequency, thereby ensuring excellent power efficiency and smaller heat generation. In the switching regulator, however, the switch noise produced in the switching of a switching element may affect the electronic apparatus as a load and peripheral equipment. Especially, an electronic apparatus with higher density tends to be more seriously affected by switching noise.
As disclosed in the references (Tetsuo Tanaka, et al. xe2x80x9cRandom Switching Control in DC-to-DC Convertersxe2x80x9d, Proc. of IEEE Power Electronics Specifications Conference, PESC ""89 (June 1989), the following method is proposed to reduce switching noise: The switching frequency is converted at random and the peak of the switching noise is diffused in the frequency range, thereby reducing the noise level. A specific method of embodying this reference is disclosed, for example, in Japanese Patent Laid-Open NO. 264849/1995.
According to the prior art introduced above, the peak of the switching noise is diffused in the frequency range. This allows noise energy to be diffused and the noise level to be reduced.
In the prior art, however, use of a simple circuit for embodiment is not sufficiently taken into account. The prior art requires use of a xe2x80x9cnoise generatorxe2x80x9d or xe2x80x9crandom signal generation circuitxe2x80x9d, but does not disclose a method for embodying them. It can be inferred that a great area is required because the random signal generation circuit is an analog circuit. Moreover, the noise of the switching regulator itself may give a serious influence. Since the switching frequency is selected at random, noise generation greatly depends on the characteristics of the switching signal occurring at random. This effect is probabilistic but not deterministic. A satisfactory effect cannot be expected at all times.
The object of the present invention is to provide an electric power converter which ensures a stable reduction of noise level at all times.
For this purpose according to the invention, in the steps of switching input voltage with a switching element 10, smoothing the voltage of rectangular wave obtained by switching with a reactor L and capacitor Cf and outputting it, the voltage obtained by dividing the output voltage is compared with the sawtooth wave output from a sawtooth wave generator 14 by a comparator 12. When the switching signal in response to the result of this comparison is applied to the switching element 10, the counter 16 is actuated synchronously with the vertex of the sawtooth wave to perform opening/closing operation of the switch SW1. The time constant of the time constant circuit comprising a R0 and C is adjusted according to whether a resistor R1 is present or not, and the signals of frequencies f1 and f2 coming out of the sawtooth wave generator 14 are sequentially switched to be sent to the comparator 12. The peak of the switching noise is diffused in the frequency range by sequential selection of switching frequencies, and noise level is reduced by diffusion of noise energy.
A first embodiment of the electric power converter according to the invention includes a signal generator for generating multiple signals having different frequencies, a signal selecting means for selecting signals of specified frequency from the multiple signals and outputting the signals sequentially in a predetermined order, a switching element for switching input voltage in response to signals selected by said signal selecting means, and a filter circuit for smoothing the output voltage of said signal generator. In this case, signals of rectangular waves having different periods can be used as multiple signals generated from the signal generator.
A second embodiment of the electric power converter according to the invention includes a switching element for switching input voltage in response to switching signals, a filter circuit for smoothing the output voltage of said switching element, a signal generator for generating multiple signals having different frequencies, a signal selecting means for selecting signals of specified frequency from the multiple signals and outputting such signals sequentially in the predetermined order, and a comparator which compares between signals selected by said signal selecting means and feedback signals obtained from the output voltage of said filter circuit, thereby generating switching signals and outputting them to said switching element.
According to the above-mentioned embodiment, signals of specified frequency are selected from multiple discrete signals sequentially in the predetermined order in the step of switching the input voltage by a switching element, and the switching element performs switching operation in response to the signals of selected frequency. This allows the peak of the switching noise to be diffused in the frequency range, and noise energy to be diffused, thereby ensuring a stable reduction of noise level at all times.
The following variations can be added when the above-mentioned electric power converter is configured:
(1) Above-mentioned multiple signals generated from the signal generator may be the signals characterized by waveforms having a certain slope.
(2) Above-mentioned multiple signals generated from the signal generator may be the signals characterized by sawtooth waveforms.
(3) Above-mentioned multiple signals generated from the signal generator are the signals characterized by triangular waveforms.
(4) The above-mentioned signal selecting means selects the signals generated from the signal generator according to the vertex of the waveform.
(5) The above-mentioned signal selecting means selects out of the signals generated from the signal generator the signals in the order from lower to higher frequencies, and then selects the signals in the order from higher to lower frequencies.
(6) The signal generator and signal selecting means may include a waveform generator for generating signals of triangular waveform and sawtooth wave, a time constant circuit for determining the period of the signals generated from the waveform generator according time constant, multiple resistors for changing the time constant, a counter which counts the signals generated from the waveform generator, compares the counted value with the preset value and outputs the switching signals synchronously with the vertex of the waveform of the signals based on the compared result, and multiple switch for adding the specified ones of the multiple resistors to the time constant circuit in response to the switching signals.
(7) Alternatively, in place of the counter, the generator and signal selecting means may include a random number generator for outputting the switching signals synchronously with the vertex of the waveform of the signals generated from the waveform generator.
(8) The above-mentioned counter may be a binary counter.
(9) The above-mentioned waveform generator, comparator, multiple switches and counter may be arranged on a single semiconductor chip.
(10) The above-mentioned waveform generator, comparator, multiple resistors, multiple switches and counter may be arranged on a single semiconductor chip.
(11) The above-mentioned waveform generator, comparator, multiple switches and random number generator may be arranged on a single semiconductor chip.
(12) The above-mentioned waveform generator, comparator, multiple resistors, multiple switches and random number generator may be arranged on a single semiconductor chip.
(13) Multiple resistors and multiple switches are each connected in series, and resistors and switches connected in series may each be connected with others in parallel.
(14) The resistance of each of the above-mentioned resistors may be different from the parallel combined resistance of other resistors connected in parallel.
(15) The resistance Ri of each of the above-mentioned resistors has a relationship of Ri=Kr.2xe2x88x92i.
(16) The above-mentioned signal generator consists of a frequency divider for dividing the original frequency.
(17) A transformer is provided between the switching element and filter circuit.
(18) The above-mentioned filter circuit has a reactor and capacitor, one end of the reactor is connected to the input side, and the other end of the reactor is connected to the ground via the capacitor as well as via the switching element.
When a binary counter is used as a counter, the space can be saved since the binary counter is made of a digital circuit. The multiple resistors added to the time constant circuit can be used as binary ladders. The binary ladder can represent 2m a resistances (a m-th power of 2) with xe2x80x9cmxe2x80x9d resistors, thereby downscaling the circuit. Thus, use of the binary counter and binary ladder allows configuration of a simple circuit, especially, configuration of a circuit suited to the formation of an integrated circuit.